Recently, a head mounted display (hereinafter referred to as HMD) has attracted public attention as an image display which can be used for representing visual information of virtual reality, remote control of a robot and so on. HMD devices of this type are required to have a high-resolution and wide-field-of-vision so as to provide video information with an enhanced impression of reality and presence. It should also be light and small so that a user may easily mount it on his or her body (particularly on the head) with no trouble. The HMD is a spectacle-like case mountable on the user's head, wherein paired back lights, liquid-crystal displays (LCDs) and lenses are disposed in the described order from the front external side of the case (FIG. 1). The viewer who wears the HMD on own head can view an image on the LCDs through the lenses in an enlarged scale, as if it were displayed on a large screen.
Recent rapid development of LCDs has provided miniaturized high-quality LCDs usable in HMD. However, an optical system that is another main component of a HMD has not sufficiently been improved with respect to saving in size and weight. HMDs have been mainly applied in the industrial fields, in particular, as devices for studying virtual reality. However, HMD devices find wide and increasing application for home-use game-machines, VTR displays and so on. Therefore, there is a keen interest in further development of more compact and light-weighted HMDs.
To solve the above-mentioned problems, the present applicant previously proposed an optical device which is filed in Japanese Patent Application No. 7-231368. This optical device will be described below.
Coordinates used for explaining the optical device is defined as follows: an axis being normal to a sheet plane and directed this side is x-axis, an axis being parallel to the sheet plane and directed upward is y-axis and an axis being normal to x-axis and y-axis and directed to the right is z-axis (FIG. 3A). A rotation angle is an angle formed between y-axis and x-y plane when observing along z-axis (FIG. 3B). The direction of clockwise rotation is defined as a positive rotational direction.
The optical device comprises a liquid crystal display (LCD) panel, a first polarizer plate, a plano-convex lens having a half-mirror coating on its convex surface, a first quarter-wave plate, a half-mirror, a second quarter-wave plate and a second polarizer plate, all of which are arranged in the described order from the incident side so that they are included in the x-y plane and are normal to the optical axis of the optical unit (FIG. 4).
The directions of the optical elements (components) are as follows (FIG. 5): the first and second polarizer plates are disposed to direct transmitting polarized light to the y-axis direction and the first and second quarter-wave plates are disposed so that their extension axes make an angle of -45 degrees with the y-axis.
Incident light from the LCD disposed at the left side enters into the first polarizer plate whereby it is polarized to the y-axis direction. The polarized light passes a half-mirror-coated plano-convex-lens and then the first quarter-wave plate whereby the plane-polarized light is converted into circularly polarized light.
The light is then divided by the half-mirror into directly transmitting light 41 and reflected light 42. The direct light is converted again by the second quarter-wave plate from the circularly polarized light into the plane-polarized light. As the respective stretch-giving axes of the first quarter-wave plate and the second quarter-wave plate have the same direction the light is the same as passed through a half-wave plate. The plane-polarized light is rotated by 90.degree. and directed in the x-axis direction.
Namely, the light alters its polarization direction by 90.degree. at every time when passing the first and second quarter-wave plates and is, therefore, absorbed by the second polarizer plate. Thus, noise light from the LCD can be shut off before reaching the user's eye.
On the other hand, the reflected light from the half-mirror passes again the first quarter-wave plate whereby it is converted to plane-polarized light directed in the x-axis direction. The plane-polarized light passes the plano-convex lens and is reflected from the half-mirror coating thereof. The polarization direction of the light is not changed by reflection. Accordingly, the light is left as plane-polarized.
The light passes for the third time the plano-convex lens and then the first quarter-wave plate whereby it is converted to circularly polarized light, it then enters into the half-mirror and is divided into two parts: one passes the half-mirror and the other is reflected therefrom. The transmitted light then passes the second quarter-wave plate whereby it is converted to plane-polarized light.
In contrast to the aforementioned direct light, the plane-polarized light is confined to the y-axis direction since it passed through the first quarter-wave plate two more times. Consequently, it passes the second polarizing plate and reaches the user's eye.
While the light passes three times through the plano-convex lens, it is subjected to refracting actions of the lenses and actions of the half-mirror-coated concave mirror. Therefore, it gains an optical power several times larger than light passed through a single plano-convex lens. In other words, it is possible to obtain the same optical power by using an optical unit having several times reduced thickness and weight.
All optical elements are bonded to each other or disposed close to each other to compose a thin optical unit (FIG. 6).
Accordingly, a HMD is particularly requested to have a reduced thickness (protrusion from the user's face) and a reduced weight loading the user's face so as to be comfortably used. The above-described optical device is effectively usable for HMDs.
However, the optical device (hereinafter described as the earlier optical device or the earlier device) filed in Japanese Patent Application No. 7-231368 has such a drawback that each optical element has not ideal characteristic and produces unnecessary noise light, decreasing a S/N-ratio of the optical system. Noise rays are produced for the following six main reasons:
(1) The first reason is birefringence (double refraction) in the plano-convex lens. This lens is preferably made of plastic material to have a reduced weight. However, many plano-convex lenses made of conventional optical plastics (e.g., acrylic resin) have birefringence caused by internal strain produced in manufacturing process. The plano-convex lens having the birefringence causes light passing therethrough to change its polarized condition. Consequently, noise rays from the LCD directly reach the user's eye and necessary rays of light (hereinafter referred to as signal light) are disturbed, resulting in decreasing a signal-to-noise ratio of an image to be displayed. PA0 (2) The second reason is a change of retardation characteristic of the first and second quarter-wave plates with an angle of light passing therethrough. When the angle of passing light differs from the designed value, the retardation value shifts from 1/4 of the wavelength. Therefore, the light can not be shut off or transmitted as designed. Consequently, similarly to the case of reason 1, some rays of light from the LCD may directly reach the user's eye as noise light. PA0 (3) The third reason is a change of retardation characteristics of the first and second quarter-wave plates for wavelengths. If the wavelength of each quarter-wave plate does not match with the designed value, its retardation deviates from 1/4 of the wavelength and the plate can not shut off and transmit light rays as designed. Consequently, a part of light from the LCD, similar to the first and second cases, directly reaches the user's eye as noise light to be further described below in detail (FIG. 14): PA0 (4) The fourth reason is that unnecessary reflection of light occurs between the plano-convex lens and the half-mirror (FIG. 22). This unnecessary reflection of light may take place at both surfaces of the first quarter-wave plate and at the user's side surface of the plano-convex lens. Noise light produced from the unnecessary reflection is polarized in the same way as the signal light, passes the second polarizer plate and reaches the user's eye. Light may be usually reflected by 5% from the surfaces, not coated with an anti-reflection film, of the plano-convex lens and the first quarter-wave plate. In this case, reflected noise rays amounts to 15% of the light. PA0 (5) The fifth reason is the light reflection from the LCD (FIG. 23). A half (50%) of light from the LCD is reflected by the plano-convex lens and returns to the LCD. If the reflected light is further reflected by the LCD, it may reach the user's eye as a noise component of signal light. Usually, the LCD with a black matrix without anti-reflection coating may reflect 25% of the reflected light, producing unnecessary reflected light, i.e., noise light that amounts 12.5% (50%.times.25%) in relation to signal light. PA0 (6) The sixth reason is such that light is reflected from rear surfaces of the second quarter-wave plate, the second polarizer plate and the half-mirror (FIG. 24). Signal light having passed through the half-mirror may be doubly reflected and become a noise light that produces a ghost image. Furthermore, external reflected light may fall on the user's eye, resulting in decreasing the contrast of a formed image. PA0 (1) An object of the present invention is to provide an optical device which uses reflecting and refracting means having a half-mirror coating, which is made of a copolymerized macromolecular material having positive and negative photoelastic coefficients relative to an internal stress. Namely, adaptive copolymerization of the macromolecular materials eliminates the possibility of double refraction. PA0 (2) Another object of the present invention is to provide an optical device mentioned above (1), wherein the first and second quarter-wave plates have refractive power in their thickness direction, which value lies between a maximal refractive index and a minimal refractive index in the plate surface direction. PA0 (3) Another object of the present invention is to provide an optical device mentioned above (1), wherein the first and second quarter-wave plates are disposed with their stretch-giving axes intersecting with each other at right angles. PA0 (4) Another object of the present invention is to provide an optical device mentioned above (1) to (3), wherein the first polarizer plate, the reflecting-refracting means, the first quarter-wave plate, the half-mirror, the second quarter-wave plate and the second polarizer plate are disposed in the described order from the incident side. PA0 (5) Another object of the present invention is to provide an optical device mentioned above (1) to (3), wherein the first polarizer plate, the first quarter-wave plate, the reflecting-refracting means, the second quarter-wave plate, the half-mirror and the second polarizer plate are disposed in said order from the incident side. PA0 (6) Another object of the present invention is to provide an optical device mentioned above (1), using, in place of conventional simple quarter-wave plates, wide-band-wave plates each of which is composed of a half-wave plate and a quarter-wave plate bonded to each other and having optical axes intersecting with each other. PA0 (7) Another object of the present invention is to provide an optical device mentioned above (6), wherein the quarter-wave plate and the half-wave plate are made of the same kind of material and at least either one of wave-plates satisfies a condition n.sub.x &gt;n.sub.z &gt;n.sub.y where n.sub.x, n.sub.z, and n.sub.y are refractive indexes in orthogonal directions and thickness direction, respectively, in a surface plane of the wave-plate. PA0 (8) Another object of the present invention is to provide an optical device mentioned above (6) or (7), wherein an angle "p" formed between optical axes of the quarter-wave plate and the half-wave plate is larger than 50 degrees and smaller than 70 degrees. PA0 (9) Another object of the present invention is to provide an optical device mentioned above (6) to (8), wherein the first polarizer plate, the reflecting-refracting means, the first wide-band-wave plate, the half-mirror, the second wide-band-wave plate and the second polarizer plate are disposed in the described order from the incident side. PA0 (10) Another object of the present invention is to provide an optical device mentioned above (6) to (8), wherein the first polarizer plate, the first wide-band-wave plate, the reflecting-refracting means, the second wide-band-wave plate, the half-mirror and the second polarizer plate are disposed in the described order from the incident side. PA0 (11) Another object of the present invention is to provide an optical device mentioned above (1) to (10), wherein the optical components are coated each at either one side with an anti-reflection coat. PA0 (13) Another object of the present invention is to provide a HMD composed of an optical device mentioned in any one of (1) to (12) above, a back light and a liquid-crystal display (LCD). PA0 (14) Another object of the present invention is to provide a HMD mentioned above (13), wherein a black-matrix contained in the liquid crystal display is provided with anti-reflection means. PA0 (15) Another object of the present invention is to provide a HMD mentioned above (13) or (14), wherein the back light has a substrate provided thereon with a light-emitting diode (LED) chip with an electrode and a fluorescent-material coat applied onto the substrate. PA0 (16) Another object of the present invention is to provide a HMD composed of an optical device mentioned in any one of (1) to (12) above and an electroluminescent display (ELD). PA0 (17) Another object of the present invention is to provide a HMD mentioned above (17), wherein a black-matrix contained in the electroluminescent display is provided with anti-reflection means.
An anisotropic body such as a quarter-wave plate may have different refractive indexes in different directions, which is expressed by using a refractive index ellipsoid that is as follows (FIG. 10):
In a plane of a quarter-wave plate, there is set a coordinate system wherein X-axis corresponds to the direction of a stretch-giving axis of the plate, Y-axis is normal to X-axis and Z-axis is normal to X-axis and Y-axis and extends in the plate thickness direction (FIG. 9).
Now suppose light propagating along Z-axis. Usually, this light is expressed as a synthesis of plane-polarized light in the direction of X-axis and plane-polarized light in the direction of Y-axis. Refractive indeces mil, of two plane-polarized rays in the quarter-wave plate made of anisotropic material are different from each other and expressed as n.sub.x and n.sub.y (FIG. 9A).
Further, light propagating along X-axis and plane-polarized in the direction of Z-axis is supposed and its refractive index is expressed as n.sub.z (FIG. 9B). An ellipsoid shown in FIG. 10 is obtained using the refractive indeces mil, according to the following expression: EQU (x.sup.2 /n.sub.x.sup.2 +y.sup.2 /n.sub.y.sup.2 +z.sup.2 /n.sub.z.sup.2)=1(Equation 1)
The ellipsoid is so called refractive index ellipsoid that can be used for approximately determining refractive indeces mil, of light rays propagating in ordinary directions except the above-mentioned directions as follows:
Light that propagates in a direction A, by way of example, is considered (FIG. 11). In this case, a line that intersects the refractive index ellipsoid and a plane passing an origin and being normal to the light propagation direction A is first.
This determined line as above-mentioned describes an ellipse whose major axis and minor axis indicate two respective axes of plane-polarized rays into which incident light is split. Refractive indeces mil, of these two plane-polarized rays are determined by lengths of the ellipse major and minor axes respectively. This refractive index ellipsoid is very useful for determining a refractive index of a retardation plate for any light passing in any direction therein.
Many conventional quarter-wave plates are sheets made of polycarbonate stretched in a certain direction. Many of these sheets have a refractive index ellipsoid which characteristics of n.sub.x &gt;n.sub.z =n.sub.y as conventional quarter-wave (FIG. 12).
The quarter-wave plate of the above-mentioned characteristic can give an adequate retardation to light passing the plate (sheet) at right angles but it alters retardation of light passing therethrough at an oblique angle. For example, a retardation for light entering obliquely into the quarter-wave plate in a plane Z-X is calculated by using the refractive index ellipsoid as follows: ##EQU1## In Equation 2, .theta. is an oblique angle measured from a normal line, d is a thickness of the quarter-wave plate (=60 micrometers), n.sub.x is a refractive index in the direction X (=1.5922), n.sub.y is a refractive index in the direction Y (=1.5900), n.sub.z is a refractive index in the direction Z (=1.5900) and L is a wavelength of the light (=520 nanometers).
The equation 2 is calculated by putting numerical values therein. As is apparent from a change of retardation with incident angles measured from the normal, retardation is decreased twice at an incident angle of 60.degree. (FIG. 13) Accordingly, an optical system using the quarter-wave plate allows slant rays to pass at a side of the visual field and fall into the user's eye and absorb a part of necessary signal light, resulting in decreasing a S/N ratio of a display image.
The earlier optical device disposes the first and second quarter-wave plates so that their stretch-giving axes match with each other and incline at 45.degree. to a transmission axis of the first polarizer plate. In this unit construction, the first quarter-wave plate and the second quarter-wave plate can cooperatively act as a half-wave plate. Consequently, the light from the first polarizer plate passes the first and second quarter-wave plates and emerges therefrom as polarized in direction normal to the transmission axis of the first quarter-wave plate.
To absorb this light that may reach the user's eye directly from the LCD, the second polarizer plate is disposed so that its transmission axis is coincident with the transmission axis of the first polarizer plate. The desired purpose can be realized if the quarter-wave plates have an ideal quarter-wave retardation for any wavelength.
However, practical quarter-wave plates vary retardation with wavelengths. By way of example, wavelength characteristics of polycarbonate usually used as a quarter-wave plate has a specific relation between a wavelength of transmitted light and retardation of the light (in percentage to wavelength) (FIG. 15).
The first and second quarter-wave plates having the characteristics are disposed with their stretch-giving axes being in line. In this case, total retardation characteristic of the first and second quarter-wave plates may remain a change of retardation depending upon wavelengths. In particular, blue light having a short wavelength suffers retardation by b 3/4 of the wavelength (FIG. 16).
Accordingly, the second polarizer plate disposed after the second quarter-wave plate can not absorb all rays of light, allowing blue noise light to reach the user's eye. As the combined retardation of two quarter-wave plates varies with wavelengths of light rays, light rays pass the second polarizer plate and reach the user's eye as noise light.
A function of a simple quarter-wave plate is described below in detail by using a Poincare's sphere. The Poincare's sphere is a system indicating polarization of light by points on a sphere (FIG. 17). If this sphere is regarded as the earth, points corresponding to the north pole and the south pole indicate counterclockwise circularly polarized light and a clockwise circularly polarized light respectively. A point on the equator relates to plane-polarized light and a longitude on the equator corresponds to twice an angle of the plane-polarized light.
Accordingly, if a longitude in a -Z-axis direction is defined 0, a point on the equator in a Z-axis direction represents a vertical straight line and -X-axis direction represents plane-polarized light in a 45-degree direction. Similarly, a x-axis direction indicates a plane-polarized light in a horizontal direction and a x-axis direction indicates plane-polarized light in a -45-degree direction. A space between the equator and the north pole represent an elliptically polarized light. Thus, any polarized light can be expressed by a point on the Poincare's sphere.
The function of a retardation plate is expressed as an effect of rotating the Poincare's sphere by a retardation value measured in terms of angles. In this case, a rotation axis is defined as a straight line jointing a center of the Poincare's sphere with a point on the equator, which corresponds to a doubled longitude degree in the optical axis direction of the retardation plate.
A simple quarter-wave plate used in the conventional optical unit is now examined (FIGS. 18A and 18B). A standard polycarbonate-made quarter-wave plate is disposed with its stretch-giving axis disposed in a -45-degree direction in order to obtain counterclockwise circularly polarized light when a plane-polarized visible light (400 nm-700 nm) falls on the plate at an angle of 90.degree. thereto.
The above-mentioned object can be realized if the retardation amount of the quarter-wave plate is always a quarter-wavelength to the incident light wavelength. However, an actually measured retardation of polycarbonate-made quarter-wave plates shows a quarter-wave retardation at the designed wave-length (500 nm) but an excess retardation with shorter wavelengths and an insufficient retardation with longer wavelengths.
The same facts can be explained by using a Poincare's sphere (FIG. 19). A plane-polarized incident light that is polarized in a vertical direction is expressed by a point A on the equator. An optical axis of a quarter-wave plate is expressed by a straight line and its activity is expressed by rotating the Poincare's sphere about the straight line B (optical axis) according to the retardation amount (90 degrees for the quarter-wave plate).
Accordingly, the point A is transferred to the north pole point C for light of the designed wavelength to form a complete circularly-polarized light, but the point A is transferred to a point D for light of shorter wavelength because of an excess retardation amount or to a point E for light of longer wavelength because of an insufficient retardation amount. Thus, light whose wavelength is other than the designed wavelength is converted to (not-circularly) elliptically polarized light.
There shows a result analyzing a ratio of circularly-polarized rays in output light of this simple quarter-wave plate (FIG. 20). This shows that the simple quarter-wave plate has different ratios of circularly-polarized rays in output light, which amount varies with different wavelengths. The earlier optical device using the simple quarter-wave plates has such a spectral transmittance characteristic of signal light where a large dispersion of visible light transmittance is observed with a particularly low transmittance of blue light and red light.
Consequently, the earlier optical device using the simple quarter-wave plates produces noise rays because the quarter-wave plates may produce elliptically polarized light.